Fluid dispensing is a critical process in a broad range of applications. Not only in medical and biomedical fields, but also in many industrial applications precise amounts of liquid have to be dispensed and/or aspirated. Hence a large number of principles have been found addressing the needs of the individual applications.
An industrially widely used dispensing approach is “Time Pressure Dispensing” (TPD). TPD is a method of dispensing liquid materials (such as surface mount adhesives and gasketing materials) that uses air pressure applied to the top of a syringe to force material through a needle. The amount of time the air pressure is applied is directly related to the amount of liquid material dispensed. Common time pressure dispensing setups are easily implemented. However, TPD allows no feedback of the liquid dispensed during the dispensing cycle. Especially syringe fill-level, viscosity of the medium, syringe to syringe variation and clogging are influencing the amount of liquid dispensed. Measures to account for those disturbance variables are matter of current research activities. (Dixon, D., 2004. Time Pressure Dispensing. White papers. Available at: https://www4.uic.com/wcms/images2.nsf/%28GraphicLib %29/Time_Pressure_Dispensing.PDF/$File/Time_Pressure_Dispensing.PDF; Chen, C.-P., Li, H.-X. & Ding, H., 2007. Modeling and control of time pressure dispensing for semiconductor manufacturing. International Journal of Automation and Computing, 4(4), pp. 422-427. Available at: http://link.springer.com/10.1007/s11633-007-0422-8).
In biomedical and laboratory applications most often volume defined pumps are used. Syringe pumps allow precise dispensing of small volumes. However, they are expensive and bulky. Alternatively, peristaltic pumps, smaller and lower in cost, can be used which create a pulsating flow, which can be problematic in many applications. Generally all pumping mechanisms can be used and observed with a flowmeter within the liquid path. The flowmeter itself often has to be calibrated and gives output correlating with the media physical properties such as viscosity.
Other examples of high-precision dispensing arrangements are disclosed in WO 9217339, in which volume is determined by counting productive and unproductive strokes of a positive displacement pump, and in U.S. Pat. No. 5,193,990, which utilises an auxiliary dispensing chamber with variable volume.
U.S. Pat. No. 6,460,730 further discloses an apparatus comprising a tank containing a fluid and connected by an inlet line to a pressurized gas supply by means of a first pressure sensor and a valve, both operatively connected to a control unit. At an outlet the tank is connected to a dispensing valve arranged and sensor arranged in the fluid feed line and also connected to the control unit so as to open a feed line for dispensing fluid from the tank.
Finally U.S. Pat. No. 5,568,882 discloses a fluid dispenser system comprising a pressure vessel, a supply tank for supplying a fluid, a bubbler sensor, a dispense pressure supply, a dispense tank for receiving the fluid from the pressure vessel and a controller, which measures a volume of fluid to be received and dispensed by the pressure vessel. A solenoid valve piloted by the controller and located downstream the pressure opens or closes the inlet/outlet lines to supply fluid to the pressure vessel or dispense fluid from the pressure vessel to the dispense tank upon gas pressure measurements in the pressure vessel.
Nevertheless, none of the above-mentioned prior art permits accurate determination of a volume of fluid dispensed or aspirated which is simple, compact, does not require a flow meter or any sensing element in the liquid path to determine precisely the volume of fluid to be dispensed, and is unaffected by partial clogging of conduits, liquid viscosity and so on.
An aim of the present invention is thus to at least partially overcome at least some of the above-mentioned drawbacks of the prior art.